A federal class action lawsuit is seeking to uncover key players behind the alleged widespread manipulation of Wall Street’s “fear index,” which plaintiffs claim has resulted in potentially hundreds of millions of dollars in losses to investors nationwide.

The complaint, filed late Friday in Chicago federal court, comes as U.S. regulators are stepping up scrutiny of the CBOE Volatility Index, or VIX, which is widely watched as a measure of investor anxiety. Last month, the Commodity Futures Trading Commission, the U.S. Securities and Exchange Commission and Wall Street’s self-regulator, the Financial Industry Regulatory Authority, all indicated they were investigating the possible rigging of the market.